How to resize a loaded image?
Type this in the blender console:

bpy.data.images["ImageName"].scale(x, y)

x, y = desired size of the image

In your case:

bpy.data.images["texture_paint_..."].scale(2048, 2048)

Old topic, but I found this code in the 2.78's API.  So we officially have a way to resize image inside blender.